The phrase “birds of a feather flock together” is used to refer to the many ways that humans who share interests, demographics, or other characteristics will naturally group themselves together in a given situation. This phrase is an example of a “metaphor of personification,” where the literal meaning, related to birds, is figuratively used to relate to humans. There is generally nothing negative or demeaning about the use of this phrase, where some other metaphors of personification can be offensive to some speakers.
Many language historians attribute the origin of the phrase to the 16th century. Originally, some writers had used a slightly alternate phrase “birds of kind and color,” which is a bit more literal than “birds of a feather,” where the word “feather” is used to denote commonality. In older forms of this phrase, old English spelling often applies.
Some uses of the phrase have been used in reference to older works. One of the most prominent is the use of this phrase in a translation of Plato’s Republic. Most researchers would agree, though, that this idiom was not explicit in the original, but rather a convention of the translator.
Along with the social use of the phrase “birds of a feather flock together,” this phrase has also started to become popular in some circles related to computer programming or IT development. In many cases, the phrase has been shortened to the abbreviation BoF. One use of this item is in describing Web forums dedicated to a particular intention or goal. In general, BoF refers to either informal work groups of people engaged in the same task, or a broader collection of developers with a common agenda or focus.
In the general category of idiom phrases in English, the phrase is often classified more precisely as a “saying,” in other words, a popular metaphor, which is also a “truism,” or re-statement of something that may often be evident. The phrase also benefits from its internal rhyme. Some also see it as mainly a descriptive phrase, when used in its original social context, that might be applied to an observed instance of people being grouped together in a certain way. In the above computer industry context, the phrase has become more a kind of “marker” for collaboration and cooperation in progressing toward particular IT goals.